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INTRODUCTION
The RadiVet has been designed and built primarily as a very compact piece of test gear for checking,
aligning and locating faults in all stages of FM receivers. The facilities provided also enable tests to be
carried out on Public Address equipment, tape decks, and AM broadcast receivers.
Basically the RadiVet consists of three independent instruments housed in one case: an AM/FM signal
generator, with crystal calibration, an audio oscillator and an oscilloscope. These three instruments,
which can be used either separately or in conjunction one another, enable every function of a
receiver to be thoroughly investigated. The interconnections required for the different tests are set
up by means of switches on the front panel.
A brief summary of the facilities of the RadiVet is given in Section 2 from which an experienced
service engineer will be able to work out his own methods of employing the instrument. Full
instructions for operating the RadiVet to obtain each of the facilities are given in Section 3, and the
application of these facilities to testing FM and AM receivers, audio amplifiers, tape recorders and
television receivers are dealt with in Sections 4 to 8.
A detailed technical description of the circuit appears in Section 9, a specification in Section 10 and
maintenance instructions in Section 11.
1. SUMMARY OF FACILITIES
The facilities provided by the RadiVet are listed below and further information on voltage ranges,
accuracy, etc. is given in Section 10.
Signal Generator
The signal generator covers the frequency ranges 0 to 15 mc/s and 85 to 100 mc/s, that is the
normal broadcast AM band and FM IF band, and the FM transmission band. The output is in each
case obtained by mixing the outputs of two oscillators, one of which may be frequency modulated
and the other amplitude modulated. The signal may therefore be un-modulated, modulated with FM
or AM or simultaneously with FM and AM at different modulating frequencies. In addition, signals in
the bands 0-30 mc/s, 45-60 mc/s, 65-80 mc/s and 195-240 mc/s are obtainable at a lower output
level (see Sect. 8).
The signal generator output is fed through a coaxial cable, to a probe unit which contains an
attenuator. Output signal levels between approximately 5 micro-volts and 50 milli-volts may be
obtained.
1.1. Wobbulator
The signal generator may be frequency modulated at 50 c/s with a frequency sweep of up to 500
kc/s. If this signal is applied to the IF section of an FM receiver, and the resultant audio signal is
applied to the `Y' plates of the crt in the RadiVet, a wobbulator display of either the IF filter
characteristics or the discriminator or ratio detector characteristics may be obtained. The effect of
trimming these circuits can be observed and the circuits aligned for optimum performance very
rapidly. If a ratio detector is being aligned, since it is possible to provide simultaneous AM at a higher
frequency, the wobbulator display so obtained enables the circuit to be set up correctly for optimum
rejection of AM.
A similar procedure, using a very much lower frequency sweep, can be applied to give the response
characteristics of a normal AM receiver IF filter.
1.2. Audio Oscillator
An audio oscillator providing a choice of eleven fixed frequencies between
40 c/s and 15 kc/s is incorporated in the RadiVet. This oscillator may be used for amplitude or
frequency modulation of the RF oscillator, as an audio output signal for tests on audio equipment and
tape recorders and to scan the `X' axis of the cathode ray tube.
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1.3. Oscilloscope
The crt has an associated DC coupled `Y' amplifier and time-base. The frequency response extends
from DC up to 1 mc/s and in conjunction with the saw-tooth time-base generator, provides a useful
oscilloscope for general waveform investigations. The `Y' attenuator is calibrated, and in conjunction
with a calibrated `Y-shift' control, enables the amplitude of any observed waveform to be measured.
1.4. DC valve voltmeter
The cathode ray tube, calibrated "Y" attenuator, and calibrated "Y-shift" control form a DC valve
voltmeter which enables voltages to be measured without placing an appreciable loss on the circuit.
This is particularly useful when measuring the potentials of valve electrodes, where an appreciable
current drawn by the measuring instrument can completely change the voltage and provide very
misleading results.
1.5. AC Valve Voltmeter
As mentioned in the paragraph dealing with the oscilloscope facility above, the amplitude of a
waveform displayed on the crt may be measured by means of the `Y' attenuator and calibrated `Y-
shift' control. The input impedance is high, as in the case of DC measurements, and the instrument
therefore operates as an AC valve voltmeter at 50 c/s and audio frequencies.
1.6. Frequency Calibrator
The instrument contains a 5 mc/s crystal oscillator that may be used to calibrate the output
frequency to a very high degree of accuracy.
1.7. Suitability for AC/DC receivers
The chassis and `earth line' of the RadiVet are isolated from earth and from the case, and the
instrument may therefore safely be used with AC/DC sets where the chassis may be `live' relative to
earth.
2. OPERATION
2.1. Initial Adjustment
The RadiVet is supplied complete and ready for use. All probes are permanently connected, and a
mains lead is provided to which a plug should be connected. The red, black and green leads are live,
neutral and earth respectively and it is important that the earth lead should be connected to ensure
correct screening.
The RadiVet is despatched with the mains tapping panel set for operation on 230 volt supplies. If
operation on supplies of different voltage is required, the fused plug on the tapping panel, which is
accessible inside the right-hand probe storage aperture, should be moved to the required tap.
2.2. Frequency Calibration
If the full accuracy of the frequency calibration of which the RadiVet is capable is to be realised, it is
necessary before use to reset the scale by means of the trimmer C75, which is located in the right-
hand compartment of the case, as shown in Fig. 2. It is also desirable, where freedom from drift is
essential, (for instance when operating on the Long and Medium broadcast bands) for the instrument
to have been switched on for a period of at least twenty minutes.
The normal procedure for crystal calibration is as follows:-
(a) Set the tuning dial accurately to the check-point nearest to the frequency at which the
instrument is to be used.
The check-points at 2.5 mc/s intervals are recommended for normal use. Additional points
which occur at very much reduced amplitude every 5/3 and 5/4 mc/s are not reliable and
should not be used unless their identity has been fully established. Both scales are linearly
calibrated in 1 mc/s steps only, whilst the number of kc/s are read off the subsidiary scale
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AIRMEC RADIVET MODEL 211 PAGE 10 OF 37
calibrated in 1 mc/s steps only, whilst the number of kc/s are read off the subsidiary scale
which is calibrated at 10 kc/s intervals and revolves once for every 1 mc/s.
Reference should be made to Fig. 4.which shows the tuning dials. The example illustrated could
be:-
4.425 mc/s
9.425 mc/s
14.425 mc/s
89.425 mc/s
94.425 mc/s
or 99.425 mc/s
depending on the setting of the RANGE switch (which determines which of the three scales is in
use) or the setting of the OUTPUT switch (which determines whether 85 mc/s is to be added to
the scale reading).
Where a crystal check-point coincides with a wanted frequency, the accuracy will be 0.01% but
between check points the accuracy will be better than ± 40 kc/s.
(b) Set the OUTPUT switch to XTAL CHECK, the GAIN switch to INT. CHECK, and the FM and AM
switches to OFF. The `X' PLATES switch may be at 50 c/s or any of the four TB ranges but not
at VAR. FREQ. since the audio oscillator is operating as an amplifier for this facility.
(c) Rotate the trimmer C75, the location of which is shown in Fig. 2., until a deflection is obtained
on the `Y' axis of the crt. The amplitude of the deflection may then be adjusted to a convenient
height by means of the RF OUTPUT control.
As the trimmer is adjusted through the correct tuning point the amplitude of the deflection will
gradually rise to a maximum and then suddenly fall to zero. If adjustment is continued in the
same direction the amplitude will rise equally suddenly and will then gradually fall off again.
The correct setting is at the zero beat between the two amplitude peaks, but it must not be
expected that this position will be completely stable and it will be sufficient if the beat is
obviously of an audio frequency.
When the OUTPUT and GAIN switches are returned to the required setting the correct output
frequency will be obtainable at Probe A (RF probe).
When the wobbulator facility is required the above adjustment should be made but before
altering the setting of the GAIN and OUTPUT switches, the FM switch and the `X' plate switch
should be set to 50 c/s and the DEVIATION control advanced. The beat note will be seen to
become localised at the centre of the trace into a small `pip'.
The frequency is now being swung at 50 c/s in synchronism with the `X' plate scan and every
time the frequency of the variable frequency oscillator is equal to that of a crystal harmonic this
`pip' is produced.
This point on the trace must now be identified since the `pip' vanishes when the instrument is
in normal use and in any case its retention would only serve to confuse the outline of any
characteristic being traced.
To do this, it is necessary to bring the `pip' exactly under the vertical graticule, using the `X'
shift control. Thereafter the point on the trace that appears under this graticule is at the
frequency shown on the dial.
The GAIN & OUTPUT switches may now be returned for normal usage. It is assumed in all the
following instructions that the frequency calibration check has previously been carried out.
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AIRMEC RADIVET MODEL 211 PAGE 11 OF 37
following instructions that the frequency calibration check has previously been carried out.
2.3. HF Signal Generator
2.3.1. Un-modulated
The signal generator frequency should be set up as indicated in Section 3.2 above, and Probe A (RF)
should be connected to the point at which the signal is required, using the shortest possible
connecting leads. The maximum output level is 50 milli-volts with the OUTPUT control fully clockwise
and the probe flying lead in the X1 socket. This may be reduced to 5 milli-volts or 500 micro-volts by
plugging the probe flying lead into the divide by 10 or divide by 100 sockets, or by a further factor of
10 by turning the OUTPUT switch to the appropriate 20 db position. With any of these output
conditions continuous decrease of output level by a further 10 times (20db) may be obtained using
the OUTPUT control. For the signal to be un-modulated the FM and AM switches should be set to
OFF.
2.3.2. Amplitude Modulation
With the signal generator set up as indicated in Section 3.3.1 above, amplitude modulation at a fixed
depth of approximately 30% may be applied by turning the AM switch either to 50 c/s or VAR. FREQ.
as required. If turned to VAR. FREQ. the modulating frequency will be that selected by the AUDIO
OSC. FREQ. switch.
2.3.3. Frequency Modulation
Normal FM, with a maximum deviation of 75 kc/s may be obtained by setting the controls as
indicated in Section 3.3. above, but with the FM switch at VAR. FREQ. NORMAL. The AM switch
should be set to OFF. The modulating frequency is the audio frequency selected by the AUDIO OSC.
FREQ. switch. Deviation is approximately 75 kc/s with the DEVIATION control at maximum
(clockwise), and may be decreased by turning this control in an anti-clockwise direction. An FM signal
having the correct BBC pre-emphasis characteristic of 50 microseconds may be obtained with the FM
switch in the VAR. FREQ. PRE-EMPHASIS position. With DEVIATION control at maximum the
deviation is then approximately 75 kc/s with a 15 kc/s modulating frequency and 14 kc/s at 40 c/s.
High deviation at 50 c/s may be obtained when using the instrument as a wobbulator but this facility
is dealt with in Section 3.5.
2.3.4. Simultaneous FM and AM
FM at 50 c/s and AM at a variable frequency or vice versa may be obtained by placing the FM and AM
switches in the appropriate positions. The former is required for checking the AM rejection of ratio
detectors which is dealt with in Section 4.3.
2.4. Audio Oscillator
The output of the audio oscillator may be obtained from Probe B when the OUTPUT control is at any
position other than XTAL CHECK. The audio frequency will be determined by the setting of the
AUDIO OSC. FREQ. switch.
The output voltage with the AUDIO OUTPUT control in the maximum (clockwise) position is
approximately 0.7 volts RMS., (2 volts peak to peak), and may be reduced by rotating the AUDIO
OUTPUT control in an anticlockwise direction. The actual output voltage may be monitored and
measured by turning the `Y' GAIN control to the INT. CHECK position, when the waveform of the
output signal may be observed and the voltage measured by the method described in Section 3.8.1.
(Note that the waveform will disappear if the Probe B leads are short circuited.)
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2.5. Wobbulator
Operation as a wobbulator involves `sweeping' the HF oscillator, that is, applying FM at a relatively
low frequency and high deviation, and applying this signal to the equipment under test. The
frequency response of the filters or discriminators will produce an amplitude modulated output from
this frequency modulated signal, and the amplitude modulation may be used to provide a `picture' of
the receiver's frequency response over the sweep-frequency range. The amplitude modulation must
first be rectified in the receiver and this is dealt with in Sections 4.1 and 5.1. (In the case of an FM
receiver this involves making the discriminator ineffective). The rectified output is taken from the
audio section of the receiver via Probe C and fed via the `Y' amplifier to the `Y' plates of the crt. The
`X' plates are connected to the modulating frequency, in this case 50 c/s, and the figure obtained on
the crt represents response plotted against frequency.
The procedure to operate as a wobbulator is therefore to adjust the signal generator as described in
Section 3.3.3, but with the FM switch at 50 c/s. The `X' plates switch should be turned to 50 c/s and
the DEVIATION control adjusted to provide the required amount of `sweep', maximum corresponding
to about 500 kc/s. The `Y' GAIN switch should be turned to one of the AC positions, the position
being chosen to give a picture of reasonable amplitude. The FOCUS, BRILLIANCE, and the `Y' SHIFT
controls should be manipulated to obtain a satisfactory picture.
The horizontal deflection of the trace represents frequency, and the frequency as set up on the
tuning control will appear accurately under the vertical centre of the graticule. As the tuning control
is rotated, therefore, the picture will move along so that any particular part of the trace may be
placed under the vertical line, and the frequency measured. Bandwidths may be determined by
measuring the frequency of the two ends of the response, and taking the difference. Accurate
frequency calibration may be obtained by checking with the crystal calibrator, as described in Sect.
3.2.
By suitable adjustment of the RF OUTPUT and the `Y' SHIFT controls the trace may be positioned so
that it lies between the 0db and infinity horizontal lines on the graticule, as shown in Fig.5. The level
at any particular frequency may then be measured by reference to the db scales on the cursor, two
of which are provided, since the trace may be inverted.
2.6. Oscilloscope
The oscilloscope may be used to observe repetitive waveforms over a frequency range of
approximately 30 c/s to 1 mc/s. Probe C should be connected to the point at which the waveform is
available and the earth clip connected to a convenient point on the chassis.
The OUTPUT switch may be in any position other than the XTAL CHECK and the `Y' GAIN switch
should be at one of the AC positions; the position being selected to provide reasonable amplitude on
the crt.
The `X' PLATES switch should be turned to one of the four T.B. positions. The position chosen will
depend on the frequency of the signal being observed and should be the one that enables a steady
picture to be obtained at some position of the TB VEL. FINE control.
When a steady waveform of reasonable amplitude is obtained, adjust the BRILLIANCE and FOCUS
controls to obtain a sharp trace and centralise the display by means of the `X' shift and `Y' shift
controls.
2.7. Valve Voltmeter DC
Voltage measurements may be made by means of the crt, the calibrated `Y' attenuator, and the `Y'
shift control. When making measurements on AC/DC receivers, care should be taken to ensure that
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AIRMEC RADIVET MODEL 211 PAGE 13 OF 37
Probe A and B are not stowed or located in any position where the leads may short circuit to earth or
be a danger to personnel.
The `X' PLATES switch should not be switched to VAR.OSC. if the OUTPUT switch is at XTAL CHECK.
With Probe C short circuited, set the trace on the central horizontal graticule with the `Y' SHIFT
VOLTS control and note the outer scale reading of the `Y' SHIFT control. Set the GAIN switch to the
desired DC range as follows:-
DC:- 0 - 20v DC x 10
0 - 60v DC x 30
0 -. 200v DC x 100
0 - 600v DC x 300
0 -2000v DC x 1000
Connect Probe C to the point at which the voltage is to be measured. A deflection of the trace
upward for a positive voltage and downward for a negative voltage will be observed.
The trace should be brought to the central line with the `Y' SHIFT VOLTS control and the reading
noted. The difference between the two readings multiplied by the GAIN setting gives the voltage.
For example:-
First Reading - 0.1v
Second Reading + 0.9v
Difference = + 1.0v
GAIN setting = x 30
Voltage = 1.0 x 30 = +30v
2.8. Valve Voltmeter AC
2.8.1. AC Voltage below 400 peak-to-peak
The connections are the same as for the preceding Section 3.7 except that it is not necessary first to
check the zero setting. Measurement may be made at frequencies between 30 c/s and 100 kc/s and
at frequencies up to 500 kc/s with reduced accuracy, using the following switch positions:-
0 - 0.7 v RMS AC x 1
0 - 2.0 v RMS AC x 3
0 - 7.0 v RMS AC x 10
0 - 40 v RMS AC x 30
0 - 140 v RMS AC x 100
First move the trace with the `Y' SHIFT VOLTS control so that its top is coincident with the central
horizontal graticule and note the reading of the INNER scale. Then move the trace until its bottom is
coincident with the central horizontal graticule and note the new reading.
Add the two readings together (ignoring the +ve and ve signs) and the result, multiplied by the
GAIN switch setting, is the RMS value of the voltage. For example:-
First reading - 0.6v
Second reading + 0.6v
Switch setting AC x 100
Voltage = (0.6v + 0.6v) x 100
= 1.2 x 100 = 120v AC RMS
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2.8.2. 50 c/s Voltage above 400 peak-to-peak
In addition to the AC readings dealt with in Section 3.8.1, higher voltages at a frequency of 50 c/s
and having no superimposed DC may be measured with the `Y' GAIN control in the DC positions. The
general procedure is the same as for 3.8.1 and the voltage ranges are:-
0 - 7v RMS DC x 10
0 - 20v " DC x 30
0 - 70v " DC x 100
0 - 400v " DC x 300
0 -1400v " DC x 1000
3. TESTS ON FM RECEIVERS
3.1. I.F. Response
Service manuals may call for either the single 'spot' frequency or wobbulator method of alignment,
but whichever method is used, it must be remembered that most modern FM receivers incorporate a
limiter. The signal input level should be kept as low as possible and must always be less than the
minimum necessary to operate the limiter. Even if a high level limiter, such as is employed with a
ratio detector is encountered, the same warning still applies.
3.1.1. Spot Frequency Alignment
Probe or Control Connecting or setting
Probe A. (RF) Signal grid of frequency changer
Probe B (Audio) Not used
Probe C (Oscilloscope) Live side of volume control (see text)
FM OFF
AM VAR. FREQ.
AUDIO OSC. Any suitable frequency; say 1 kc/s
GAIN AC x 1
PLATES 50 c/s or any of the 4 TB positions
TUNING * Receiver intermediate frequency
RF FREQ. RANGE as required
OUTPUT Minimum possible consistent with
reasonable deflection of the crt trace
* Some types of receiver use an IF which is higher than 15 mc/s. Since the output of the RadiVet is
obtained by mixing the output from a variable frequency oscillator with `preferred' harmonics from a
5 mc/s crystal oscillator, alternative output frequencies of 15 to 20 and 20 to 25 mc/s are present at
Probe A when the RANGE switch is set at 10 to 15mc/s.
For 15 to 20 mc/s use the 5 to 10 mc/s scale and for 20 to 25 mc/s the 10 to 15 mc/s scale. In each
case add 10 mc/s to the scale setting. Signals in the band 15 to 20 mc/s are approximately 12 db
below the main output signal levels and the amplitude of the signals in the 20 to 25 mc/s band is, of
course, still lower. No reduction in accuracy is entailed when these higher output frequencies are
used.
When called for in the service manual, the local oscillator of the receiver should be rendered
inoperative; this can usually be effected by connecting the oscillator grid to earth.
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Remove the core from the secondary tuned circuit of the discriminator or the ratio detector and with
the RadiVet connected as above, adjust all the IF cores for maximum deflection of the crt trace.
Should the initial signal level be insufficient, due to a receiver fault, it may be increased by
disconnecting the RF tuned circuit from the signal grid of the frequency changer, but this should not
normally be necessary. The level of the signal on the crt trace can be increased by connecting Probe
C to some point further along the AF amplifier chain (e.g. the grid of the power output stage) whilst
any necessary reduction of the trace amplitude should, wherever possible, be effected by reduction
of the signal input level.
Correct limiter action will be shown by a decreased deflection for an increased signal input beyond
the limiter threshold level. For this check, it will generally be necessary to set the GAIN switch to
some higher attenuation value than X1 in order to prevent overloading the `Y' amplifier.
3.1.2. Wobbulator Alignment
Probe or Control Connecting or Setting
Probe A (RF) Signal grid of frequency changer
Probe B (Audio) Not used
Probe C (Oscilloscope) Live side of volume control
FM 50 c/s
DEVIATION Maximum (fully clockwise
AM OFF
GAIN AC X1
`X'-PLATES 50 c/s
TUNING (RF Frequency Range) Receiver intermediate frequency as required
Minimum possible consistent with reasonable
OUTPUT deflection of crt trace
Remove the core from the secondary tuned circuit of the discriminator or ratio detector and adjust all
cores for maximum deflection of the crt trace. Since the shape of the response will vary with the
design of the IF stages, some curves will not appear as shown in Fig. 5, but will have sharp peaks. In
the latter case an additional check can be made if a tuning indicator is fitted, as this will generally
show maximum deflection when the cores are correctly adjusted. The final curve should, in all cases,
be symmetrical about the vertical graticule and in some cases the service manual will illustrate the
type of curve which should be obtained.
Correct limiter action will be shown by a flattening of the peak of the response curve when the input
signal is increased beyond the limiter threshold level.
3.2. Discriminator or Ratio Detector Response
For this operation, service manuals may again call for either the `Spot' frequency or the Wobbulator
method of alignment. The latter method is to be preferred however, since it offers a simultaneous
check of correct alignment and linearity.
3.2.1. `Spot' frequency alignment
Probe connections and control settings required are those tabulated for Section 4.1.1. The RadiVet
OUTPUT control should then be adjusted until the signal input is just less that that necessary to
operate the limiter (i.e. deflection still increases with increased input).
If the core of the discriminator or ratio detector secondary coil is replaced and screwed in, the
deflection of the crt trace will rise to a peak, decrease slightly, rise to a further peak and then return
to zero. The core is correctly adjusted when the bottom of the trough, between the two peaks, is
reached. When the receiver is fitted with a tuning indicator the correct core position will generally
coincide with an indication of correct tuning.
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AIRMEC RADIVET MODEL 211 PAGE 16 OF 37
3.2.2. Wobbulator Alignment
Connect the RadiVet as tabled in Section 4.1.2 and replace the core of the discriminator or ratio
detector secondary coil. As the core is screwed in, at some point, providing no faults exist in the
receiver, a trace similar to that shown in Fig. 3a will be obtained. The portion of the trace which
crosses the horizontal axis will generally be linear and the two `humps' of opposite polarity should be
equal in amplitude. When the alignment is correct the vertical graticule, which corresponds to the
exact IF, will bisect the horizontal distances between the two `humps'.
Should the servicing data call for a specific bandwidth between either extremities of the linear portion
of the response - or the two `humps'- the bandwidth may be measured by noting the scale readings
when one, and then the other limit required, is placed under the vertical graticule. The bandwidth is
the difference between these two readings.
3.3. AM Rejection
Probe or Control Connection or Setting
Probe A (RF) Signal grid of frequency changer
Probe B (Audio) Not used
Probe C (Oscilloscope) Live side of volume control
FM 50 c/s
DEVIATION Maximum (fully clockwise)
AM VAR.FREQ.
AUDIO OSC. Any suitable frequency; say 3 kc/s
GAIN AC X1
`X' PLATES 50 c/s
TUNING
Receiver intermediate frequency as required
RF FREQ. RANGE
OUTPUT Minimum possible consistent with
reasonable deflection on crt trace
AM rejection is achieved in most receivers by limiter action, but when the incoming signal is below
the limiter threshold value and a ratio detector is in use, some additional AM rejection is afforded.
Ratio detectors will reduce AM at all frequencies and will eliminate it at the frequency at which the
currents in the two diodes are equal. Discriminators however, afford no appreciable AM rejection and
are usually preceded by a low-level limiter stage.
The efficiency of the AM rejection can be assessed by means of the response curve which will be
obtained when the RadiVet is connected as tabled above. The curve will appear as shown in Fig. 3(b)
or 3(c). It can be seen that there is one point on each curve where AM rejection is complete but in
Fig. 3(c) this occurs at the incorrect tuning point. Any receiver which presents a response curve of
this type would be liable to interference of amplitude modulated form (motor ignition, un-suppressed
motors etc.) even when correctly tuned for minimum distortion and, in the case of a ratio detector,
maximum output.
The cause of such a characteristic is differing AF paths for the two diodes. Some manufacturers
insert small resistors in series with each diode load so that one, or the other, may be short circuited
to bring the circuit nearer balance. In some receivers these adjustable resistors may take the form of
a potentiometer (with earthed slider) inserted in series with the bias load. If no such adjustment is
provided, one or both of the diode load resistor values maybe outside the limits laid down in the
service manual. The resistors should be checked and, if necessary, replaced.
In extreme cases, the fault may be due to an out of balance condition of diode impedance caused by
one diode losing emission, or the use of unmatched crystal diodes.
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3.4. RF alignment
Since there is considerable variation in the manner in which the RF circuits of FM receivers are tuned,
the specific alignment instructions in the manufacturers service data sheets must always be followed.
Probe or Switch Connection or Setting
Probe A (RF) Aerial input (or at point stated in service
data sheets.)
Probe B (Audio) Not used
Probe C (Oscilloscope) Input to 1st. AF stage
FM VAR. FREQ. NORMAL
DEVIATION As required see text
AM OFF
AUDIO OSC. Frequency required. Say 1 kc/s
GAIN AC Position as required to prevent
overloading of `Y'- amplifier
X PLATES 50 c/s or any of the four TB ranges
TUNING Required alignment frequency
RF FREQ. RANGE + 85 mc/s (Normal or 20 db as
required
OUTPUT As required See text
Using the above connections, adjust all trimmers etc. as laid down in the servicing instructions. The
amplitude of signals on the crt should be adjusted as required by the setting of the GAIN switch and
the receiver volume control. The setting of the DEVIATION control will also affect the amplitude of
the crt display, but this control should be set as required by the service data sheets. Maximum
deviation of 75 kc/s is obtained when the control is fully clockwise.
When aligning RF stages, great care must be taken to ensure that the signal input level is always less
than that necessary to operate the limiter. After each trimmer has been adjusted, check that a slight
reduction in the RadiVet output does, in fact, reduce the amplitude of the crt display. If this is not so,
reduce the RadiVet output accordingly and re-check the trimmer setting.
3.5. Overall check and de-emphasis
100% modulation on the existing BBC system refers to 75 kc/s deviation. With the pre-emphasised
transmission characteristic of 50 micro-seconds which is used, the modulation signal level which is
necessary to provide 75 kc/s deviation at 15 kc/s will only provide 14 kc/s deviation at 40 c/s,
and this accent on the higher frequencies has to be eliminated by the de-emphasis circuit on the
receiver. In order to check that this de-emphasis is being correctly applied in the receiver, the
RadiVet can be frequency modulated with the correct pre-emphasis characteristic by setting the FM
switch to VAR. OSC PRE-EMPHASIS.
Probe or switch Connection or setting
Probe A (RF) Aerial input (or at point stated in service data
sheets)
Probe B (Audio) Not used
Probe C (Oscilloscope) Loudspeaker terminals
FM VAR. FREQ. PRE-EMPHASIS
DEVIATION Maximum (fully clockwise)
AM OFF
AUDIO OSC. Frequency required (see text)
GAIN AC position as required to prevent overloading
the `Y' amplifier
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AIRMEC RADIVET MODEL 211 PAGE 18 OF 37
X-PLATES 50 c/s or any of the 4 TB ranges
TUNING Required alignment frequency
RF FREQ. RANGE +85 mc/s (normal or 20 db as reqd.)
OUTPUT Sufficiently high to just operate limiter stage
With the RadiVet connected as above, advance the RF OUTPUT control until the amplitude of the crt
trace stops increasing with increased input. Continue to turn the control for approximately 1/8" past
this point to ensure that the signal input is operating the limiter without overloading the receiver.
Adjust the GAIN switch and receiver volume control to obtain a crt display of about 1" in height and
note any changes in display amplitude as the AUDIO OSC. control is switched over the frequency
range of the audio amplifier.
A level output indicates correct de-emphasis whilst a lack of de-emphasis results in an output which
increases with frequency. If the output decreases with increased frequency, the cause may be too
much de-emphasis, but in any case the checks enumerated in Sections 6.1 and 6.2 (pages 19 & 20)
should be carried out to prove that the AF amplifier response is normal. This provision is made
because the setting of tone controls or a faulty amplifier can materially alter the overall
characteristics.
4. TESTS ON AM RECEIVERS
The output frequency specification limits of ± 40 kc/s assume great importance at the low
frequencies required for the correct alignment of IF stages and the Long and Medium wavebands of
AM broadcast receivers. It is therefore recommended that an early opportunity be taken to check the
scale accuracy at low frequencies. This should be done by calibrating the instrument at 0 and 1.25
mc/s with the Crystal Calibrator (as outlined in Sect. 3.2) and comparing the output frequencies with
the tuning scale readings of a receiver of known accuracy. If the more common calibration
frequencies are checked and the corrected scale readings noted, these frequencies can always be
reproduced with a very high degree of accuracy.
The crystal calibration must be accurately carried out each time these low frequencies are required.
4.1. IF Response
Connect the RadiVet as tabulated in Sect. 4.1.1 and align all IF cores and trimmer capacitors for
maximum output.
If it is desired to obtain a wobbulator display of the IF response, connect the RadiVet as in Sect.
4.1.2., set the DEVIATION control initially to about 1/5th of its total clockwise movement and adjust
as required. A trace similar to that shown in Fig.5 will probably be obtained, but reference should be
made to the service data sheets to find the correct response. A much smoother control of deviation
can be obtained if the `X'-plates are switched to VAR.OSC., the FM switch is set at VAR.OSC.-
NORMAL, and the AUDIO OSC. set to a frequency between 40 and 400 c/s. Under these conditions
some receivers may present a double trace because of phasing differences and the absence of
blackout on the trace return, but no confusion should arise regarding the shape of the response
curve.
Bandwidth measurements may be made in the manner outlined in Fig. 5 but the value obtained will
generally be of the order of 5 to 10 kc/s instead of 200 kc/s as quoted in the example.
4.2. RF Alignment Check
The frequency accuracy of the RadiVet after crystal calibration is high enough for the RF alignment of
AM receivers on the short wave bands to be carried out. At the lower frequencies however, (that is in
the medium and long wavebands) the RadiVet scale must be checked as indicated in Sect. 5.
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AIRMEC RADIVET MODEL 211 PAGE 19 OF 37
The RadiVet is then connected as tabulated in Sect. 4.1.2 and the RF circuits aligned as described in
the manufacturer's instructions.
An alternative method for calibrating the RadiVet at low frequencies is given below. This is very
simple but only provides a series of accurate spot frequencies. Set the RadiVet to 1.25 mc/s as
checked by the crystal calibrator and connect as follows:-
Probe or Switch Connection or Setting
Probe A (RF) Aerial input
Probe B (Audio) Not used
Probe C (Oscilloscope) Across loudspeaker terminals
FM OFF
AM VAR. FREQ.
AUDIO OSC. Any suitable frequency; say 1 kc/s
GAIN AC position as required to give a
display of reasonable amplitude
without overloading the `Y' amp.
`X' PLATES 50 c/s or any of the first 4 TB
positions
TUNING 1.25 mc/s
RANGE 0 5 mc/s
OUTPUT Maximum
Tune the receiver to 1.25 mc/s until the RadiVet signal is heard. If the RadiVet is now tuned through
the frequency range 100 kc/s to 1.25 mc/s a signal will be heard at each of following frequencies:-
125, 139, 156.3, 178.6, 208.3, 250, 301.25, 416.6, and 625 kc/s.
Once the positions of these spot frequencies on the RadiVet tuning dial have been determined, any
of them may be used for the alignment procedure. For instance, if the manufacturer specifies
alignment at 100 kc/s and 600 kc/s, the 125 kc/s and 625 kc/s signals could be employed.
4.3. Overall Check
This test is an extension of Sect. 5.2 and the RadiVet should be connected as tabulated for that
section. Set the TUNING control to the required frequency and switch the AUDIO OSC. from 40 c/s to
15 kc/s. The output amplitudes at each frequency should be noted and plotted to illustrate the
overall frequency response of the receiver.
5. AUDIO AMPLIFIERS
The tests outlined in Sect. 6.1 & 6.2 can be applied to any audio amplifier, either as a whole or to an
individual stage. The AF stages of commercial receivers are not normally subjected to rigorous
response checks, but where the overall performance of a receiver is indifferent, due perhaps to a lack
of either treble or bass response, overall distortion and response tests can be informative.
Detailed information on the response of audio stages is not normally given in the servicing
instructions, but it will be found that the audio stages of AM receivers will generally have a range of
about 400 c/s to 5 kc/s, whilst some FM receivers might have a very much wider range. When
making overall response checks on any receiver the limitations of the audio amplifier must always be
borne in mind if a true picture of the performance is to be obtained.
`Hi-Fi' equipment and public address systems will probably reproduce the full range of frequencies
from 400 c/s to 15 kc/s with little or no distortion.
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AIRMEC RADIVET MODEL 211 PAGE 20 OF 37
Information on the characteristics of any type of tone control can be obtained by performing these
tests with the tone control in its extreme positions and plotting the curves so obtained.
5.1. Distortion check and phase-shift measurement
Probe or Switch Connection or Setting
Probe A (RF) Not used
Probe B (Audio) Input to first AF stage
Probe C (Oscilloscope) Loudspeaker terminals
OUTPUT Any position except XTAL CHECK
AUDIO OSC. Frequency required. See text
AUDIO OUTPUT As required
GAIN AC position as required to give a crt
display of reasonable amplitude without
overloading the `Y' amp.
`X' PLATES VAR. OSC.
Connect the RadiVet as tabled above and switch the AUDIO OSC. to 40 c/s. If the output is pure, an
inclined linear trace, as shown in Fig. 3d will be obtained. (A rapid check, to see that the RadiVet is
not causing distortion, can be made by switching the `Y' GAIN to INT:CHECK. A straight line will
normally be seen regardless of the AUDIO OSC. frequency). By checking the non-linearity of the
trace at each frequency from 40 c/s to 15 kc/s, a thorough check can be made of the distortion
characteristics of the amplifier.
If harmonic distortion is present, the tips of the trace will bend towards the horizontal as shown in
Fig.3g whilst overloading the amplifier will result in a trace similar to Fig. 3f. Severe overloading will
cause both ends to bend sharply in the manner of Fig. 3h. If overloading is experienced before
maximum rated output is reached possible causes are faulty supplies, a low emission valve or faulty
biasing conditions.
The output in watts may be determined by measuring the RMS deflection of the trace and calculating
as follows:-
(Output voltage)²
Watts =
Loudspeaker impedance
If the maximum output is too loud for continuous testing, the loudspeaker should be replaced by a
high-wattage resistor of the same value.
If the trace opens into an ellipse, as shown in Fig 3e, the cause is phase-shift between the input and
output of the amplifier. A horizontal or vertical ellipse denotes 90° phase-shift and in other cases, the
phase angle may be found by carefully centralising the ellipse about the central point O, as shown in
Fig. 3e, and measuring distances OP and OQ. The phase angle may then be calculated as follows:-
OP
Sin q = where q is the phase angle
OQ
(NB: 180° phase-shift will produce a straight trace inclined from top left to bottom right of the crt.)
5.2. Overall response check
Connect the RadiVet as tabulated in Sect. 6.1 and with the `X' PLATES set at 50 c/s or any of the TB
positions, measure the amplitude of trace deflection on the crt at each frequency from 40 c/s to 15
kc/s. If these results are plotted they will illustrate the overall response characteristic of the amplifier.
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AIRMEC RADIVET MODEL 211 PAGE 21 OF 37
If the characteristic has an unexpected falling tendency at either end, or in the middle, the position
of any tone control should be checked. If their setting is not the cause of the unusual characteristic,
examine each audio stage individually until the cause of the fault is located.
6. TESTS ON TAPE RECORDERS
The RadiVet is extremely useful for assessing the performance of, setting up and locating faults in
magnetic tape recorders. When manufacturers' instructions or service data sheets are available they
should be followed as closely as possible. The following instructions are intended to supplement
those given for any particular machine, and in the absence of any specific servicing data, they will
serve as a guide to the procedure which should be adopted.
6.1. Bias
The bias waveform must be completely free from distortion or noisy recordings may result. The bias
frequency is less important: it usually lies between 40 kc/s and 60 kc/s, and quite wide latitude is
permissible. Incorrect bias amplitude can cause distortion, low output and poor signal/noise ratio.
6.1.1. Bias Waveform
Use the oscilloscope facility of the RadiVet and connect Probe `C' to the bias supply point of the
`record' head.
Any distortion of the bias oscillator waveform should be remedied until a pure waveform is obtained.
A common cause of distortion is over-driving the bias oscillator valve and if distortion is apparent this
possibility should be examined before more complex measurements are made.
6.1.2. Bias Frequency
Use the oscilloscope facility of the RadiVet with the `X' PLATES switch set to VAR. OSC. Under these
conditions, a Lissajou pattern (Fig. 7) will be obtained and the AUDIO OSC. switch and the recorder
bias frequency control should be adjusted in conjunction with each other until the pattern is
stationary. By cross-reference between the pattern and the table below, the exact frequency of
oscillation can be determined and hence the oscillator adjusted to the nearest frequency
to the one specified in the instructions.
Bias Frequency Loops AUDIO OSC. Frequency
40 5 8
42 2x7 12
44 2 x 11 8
45 3 15
48 4 12
50 10 5
52 2 x 13 8
54 2x9 12
56 7 8
60 4 15
6.1.3. Bias Amplitude
The correct level of bias is usually specified as being a level slightly greater than that required to
obtain the maximum output for a given signal input. If the required level is known, the bias level can
be measured using the AC valve voltmeter facility and may then be adjusted as required.
If the correct bias amplitude is unknown it may be ascertained as follows:-
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AIRMEC RADIVET MODEL 211 PAGE 22 OF 37
Connect Probe `B' to the `record' input terminals, set the AUDIO. OSC. to 400 c/s, adjust the AUDIO
OUTPUT and `record' level controls to the normal recording level and measure the Bias Amplitude by
means of the AC Valve Voltmeter facility. Record short lengths of tape at different bias levels and
tabulate bias levels against footage indicated readings.
Play back the recorded lengths using the AC Valve Voltmeter facility to measure the output from each
length of tape and tabulate the results of output against footage indicator readings.
If these two sets of results are combined to plot the curve of output amplitude with reference to bias
level, the correct bias level can be determined from the curve, as illustrated in Fig. 6.
6.2. Recording
The correct recording level, which is normally taken as being half the signal input necessary for tape
saturation, is important if the optimum signal/noise ratio is to be obtained whilst still permitting the
recording of reasonable peaks without distortion.
If true reproduction is to be obtained using a `playback' amplifier having the accepted 6db increase
per octave below 1 kc/s, the record characteristics must be correct. It is normally level up to the
anode of the final amplifier, though in some cases pre-emphasis may be applied to frequencies above
2 kc/s.
6.2.1. Recording level
Bias Frequency Loops AUDIO OSC. Frequency
40 5 8
42 2x7 12
44 2 x 11 8
45 3 15
48 4 12
50 10 5
52 2 x 13 8
54 2x9 12
56 7 8
60 4 15
Using the above connections record short lengths of tape at different record inputs, checking that the
crt display shows no signs of distortion, (i.e. the `record' amplifier is not being overloaded) and
tabulate the levels used against footage indicator readings. The maximum input that should be used
is just below the level that overloads the `record' amplifier.
When this has been completed, disconnect Probe B, connect Probe C to some suitable point in the
playback amplifier chain and play back the tape. Tape saturation will cause distortion of the
waveform displayed on the oscilloscope, and the correct `record' level can thus be identified.
6.2.2. Record characteristics
When making this check the tape may, if desired, be removed from a machine that is not fitted with
an automatic stop.
Connect the RadiVet as tabled in Sect. 7.2.1 and adjust the AUDIO OUTPUT and `record' level
controls until the correct level is indicated. Set the AUDIO OSC. at 40 c/s and measure the amplitude
of the deflection of the crt trace. Repeat this at all frequencies from 40 c/s to 15 kc/s and plot the
results to illustrate the `record' characteristics.
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AIRMEC RADIVET MODEL 211 PAGE 23 OF 37
6.3. Overall characteristics
Connect Probe B to the `record' input terminals and record a length of tape at each playing speed as
follows:-
At an input frequency of 400 c/s adjust the `record' level to slightly less that one half of its correct
value and then record all frequencies from 40 c/s to 15 kc/s. Tabulate frequency against footage
indicator readings.
Play back each length of tape measuring the output amplitude at each frequency by means of the AC
valve voltmeter facility and tabulate the results of output against footage indicator readings.
Combine these two sets of figures to give a table of output amplitude with reference to input
frequency and plot the curve. The play-back characteristics should, in general, have a level frequency
response as follows:-
Approximately 40 c/s to 5 kc/s at 3 ¾ ips tape speed.
= 40 c/s to 12 kc/s at 7½" ips tape speed.
= 40 c/s to 15 kc/s at 15" ips tape speed.
6.4. Wow, Flutter and Tape Drop-out
Record a length of tape at 400 c/s with Probe B connected to the input terminals and the AUDIO
OUTPUT and `record' level controls adjusted to give the correct level.
Play back the recording and examine the output, both aurally and by means of the oscilloscope
facility of the RadiVet. If the recorder suffers from wow or flutter, the note will be heard to vary in
frequency and the synchronisation on the oscilloscope will be erratic. If this is so the capstan should
be checked for eccentricity.
Random variations in amplitude (sometimes known as `drop-out') will be immediately apparent from
the above. The cause is almost always a faulty tape, and the imperfect section should be excised.
6.5. Tape slip
Record a length of tape as in Sect. 7.4, but at a low frequency (say 60 c/s). Play back this recording
and monitor the output using the oscilloscope facility with the `X' PLATES set at VAR. OSC. and the
AUDIO OSC. at 60 c/s. An almost stationary ellipse should be obtained. Any sudden change in the
location of the ellipse is due to tape slip which is generally caused by inadequate pressure from the
pinch roller.
7. TESTS ON TELEVISION RECEIVERS
The TeleVet Type 877 is designed solely for testing television receivers and the RadiVet is a
complementary instrument which was not designed for this purpose. The RadiVet can, however,
provide a number of useful facilities in this direction.
A large number of manufacturers instruction sheets specify the `spot' frequency method of alignment
and outputs in the following ranges can be obtained from Probe A at an amplitude of approximately
50mv:-
0 - 15 mc/s in 3 x 5 mc/s bands
45 - 60 mc/s in 3 x 5 mc/s bands: (Output switch to +85 mc/s)
The two above ranges can be amplitude modulated at 50 c/s or with any of the audio oscillator
frequencies, if required.
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AIRMEC RADIVET MODEL 211 PAGE 24 OF 37
The following ranges of weaker signals can be obtained either un-modulated or frequency modulated
at 50 c/s or any of the audio oscillator frequencies.
0 30 mc/s in 3 x 10 mc/s bands: This is the 2nd. harmonic of the 0 15 mc/s
(FM or AM) range.
65 80 mc/s in 3 x 5 mc/s bands: Output switch to +85 mc/s. The level of signal is
(FM only) greater than that of the normal output ranges.
195-240 mc/s in 3 x 15 mc/s bands: Output switch to +85 mc/s. This is the 3rd.
harmonic of the 65-80 mc/s range, but it may be
of sufficient amplitude to be of use on Band lll
receivers.
In all cases, provided that calibration adjustments are carried out as specified in Sect. 3.2, these
ranges are of the same high order of frequency accuracy as the normal output range of the RadiVet.
7.1. Alignment of vision channel.
Connect Probe A to the required signal injection point of the receiver and with Probe C connected to
some point between the vision detector and the modulating electrode of the crt, carry out the
aligning procedure as detailed in the service data sheets.
When amplitude modulated signals are injected, the AC Valve Voltmeter facility should be used to
monitor the amplitude of signal received at the vision amplifier, whilst if un-modulated signals are
injected, the DC Valve Voltmeter facility should be used to measure the changes in DC level caused
by signal injection and trimming adjustment.
7.2. Alignment of sound channel.
Spot frequency alignment, using amplitude modulated signals only, may be carried out as in Sect.
8.1., but with Probe C connected to any required point between the sound detector and the
loudspeaker.
The audio circuits can, of course, be checked as detailed in Sects. 6.1 and 6.2.
7.3. Time base checks.
Since the frequency response of the Oscilloscope `Y' amplifier extends to at least 1 mc/s, line and
frame time-base waveforms may be examined without introducing any appreciable distortion by
connecting Probe C to the required point and setting the GAIN switch to the AC position which gives
a display of reasonable amplitude. The waveform can be resolved by switching the `X' PLATES to one
of the TB positions as required and adjusting the TB VEL. FINE control until a satisfactory display is
obtained.
8. DESCRIPTION OF CIRCUIT
8.1. General
The circuit diagram of the instrument is shown in Fig. 1a/1b (in the appendix) and the list of
components in Table 1.
Figure 2 shows the location of the major components.
The circuit consists essentially of a signal generator, audio oscillator, oscilloscope and crystal
calibrator and each of these basic units is described in turn below.
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AIRMEC RADIVET MODEL 211 PAGE 25 OF 37
8.2. RF Signal Generator
The double triode valve V1 forms the variable frequency oscillator. One half is the reactor valve V1a
and the other half is the tuned anode oscillator. The basic range is 65 70 mc/s. Two other ranges
are obtained by means of the RANGE switch S7, which connects shunt inductors and capacitors to
give ranges 70 75 mc/s and 75 80 mc/s. The main tuning capacitor is C7 and this has been made
linear in frequency by an adjustable capacitor C8 which is set up at each megacycle calibration point
in the factory. Each calibration adjustment is accessible in turn through the plugged hole in the front
panel as the dial is rotated. Re-adjustment should not be attempted without the correct equipment.
In order to be able to adjust the oscillator against the crystal calibration, a trimmer C75, which is
accessible at the right-hand side of the case, has been provided.
A proportion of the common anode signal voltage of the valve V1 is fed via the quadrature network
formed by the anode to grid capacitance of V1a and R1, to the grid of V1a. V1a signal current is then
in quadrature with the current of V1b. Both these currents pass into the common tuned circuit and
the frequency is dependant on the proportion of these currents.
The amount of quadrature current is controlled by altering the bias of V1a by means of AF signals fed
to the grid via the DEVIATION control R4. This results in frequency modulation of the variable
frequency oscillator.
The signal is then fed to the pentode buffer amplifier V2a. This amplifier is flatly tuned over the
range 65 80 mc/s and shares a common anode circuit with the crystal harmonic amplifier V3a.
The oscillator valve V5b is a pentode which generates the fundamental crystal frequency of 5 mc/s
by using the screen as the anode of a triode oscillator. The anode circuit L7 is tuned to 65 mc/s,
(that is the 13th harmonic of the crystal). A certain amount of regeneration takes place via C57. L7 is
normally tuned with circuit stray capacitance, but when the OUTPUT switch S4 is in either of the +85
mc/s positions, L7 is shunted by C76 so that it tunes to 20 mc/s (that is the fourth harmonic of
the crystal).
When the OUTPUT switch is in the central position XTAL CHECK, the damping resistor R25 flattens
the tuning of L7 so that preference is not given to any crystal harmonic.
The output of the variable frequency oscillator (65-80 mc/s) therefore appears at the anode of the
crystal harmonic amplifier together with either of the following:-
(a) Preferred 13th harmonic (65 mc/s) - NORMAL
(b) All crystal harmonics - XTAL CHECK
(c) Preferred 4th harmonic (20 mc/s) - +85 mc/s
These added outputs when fed to the mixer valve V2b will result in the following:-
(a) VFO minus crystal 65 mc/s, giving 015 mc/s in three ranges 0-5, 5-10 and 10-15 mc/s.
(b) Each crystal harmonic from the 13th to the 16th will produce an audio beat at 5 mc/s intervals
so that the frequency of the VFO may be checked at 5 mc/s intervals. Additional beats may be
found at 2.5 mc/s or even 1.667 mc/s and 1.25 mc/s intervals due to the second, third and
fourth harmonics of the VFO beating with higher order harmonics of the crystal.
(c) VFO plus crystal 20 mc/s giving 85- 100 mc/s in three ranges 85-90, 90-95 and 95-100 mc/s
Each of the outputs is controllable in amplitude by the RF OUTPUT control R22, which varies the
anode voltage of the mixer valve V2b. The additional tuned circuit L10, C79 ensures that enough of
the 20 mc/s signal is available at the mixer grid.
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AIRMEC RADIVET MODEL 211 PAGE 26 OF 37
The output signal passes via C17 and S4a (which imposes 20 db attenuation in two of its settings) to
the output Step Attenuator housed at the end of RF OUTPUT lead (Probe A).
It is possible to pass an audio frequency or 50 c/s signal via the `AM' switch S2 to the screen of the
crystal harmonic amplifier V3a. This will result in amplitude modulation to a constant modulation
depth of the output on all ranges.
Frequency modulation at an audio frequency or 50 c/s can be passed to the DEVIATION control via
the FM switch S1. C6 and R8 form a pre-emphasis network so that the audio signals may be pre-
emphasised to the same degree as the BBC transmissions. This means a maximum deviation of 14
kc/s at 40 c/s rising to a maximum of 75 kc/s at 15 kc/s modulating frequency. De-emphasis
should take place in the receiver.
8.3. Audio Oscillator
Triode valve V3b operates as an audio oscillator, feedback from anode to grid being applied by the
phase reversing auto-transformer T1. Frequency selection is by means of the switch S5a which
determines which of 11 `Wien bridge' networks is in use. The amplitude of oscillation is limited by the
power sensitive thermistor R36 which loads the circuit in such a way that the output at each
frequency is limited to the same value and is undistorted by the valve.
Apart from providing outputs for FM and AM the circuit also p